The invention is based on a hydraulic vehicle brake system as defined hereinafter.
U. K. Patent 2,168,771 discloses a vehicle brake system with a brake booster supplied from an auxiliary pressure source; a master brake cylinder actuatable by the brake booster; wheel brakes; and an apparatus for preventing the danger of wheel locking and the attendant skidding during braking as well as for limiting drive slip. This apparatus has a first valve assembly with valves for blocking connections between the master brake cylinder and the wheel brakes, and a second valve assembly made up of a plurality of valves and used for controlling the brake booster in both the anti-skid and drive slip limitation situations. The design of the second valve assembly is disadvantageously complicated and expensive, so that installing a brake system of this type in otherwise relatively inexpensive vehicles makes them notably more expensive.
A vehicle brake system of this generic type, known from U.S. Pat. No. 4,589,706, dispenses with control of the brake booster by means of a complicated valve assembly during anti-skid operation and drive slip limitation. Second valves are therefore disposed between the first, blockable valves, which communicate with brake pressure connections of the master brake cylinder, and wheel brakes; these second valves are embodied as multi-position valves, which are complicated and expensive, and are capable of blocking connections between the first valves and the wheel brakes and of connecting the wheel brakes with a low-pressure supply container of the auxiliary pressure source. Further valves are connected to the brake booster and to the auxiliary pressure source, located between the first valves and the second valves and communicate with the second valves. In order to prevent the danger of skidding, these further valves serve to replace the quantities of pressure fluid that were diverted from wheel brakes to the low-pressure supply container of the auxiliary pressure source with such pressure fluid drawn from the brake booster, while to dissipate excessive drive torque they serve to introduce quantities of pressure fluid from the auxiliary pressure source into the wheel brakes of driven wheels exhibiting a tendency to spin. In this vehicle brake system as well, the overall engineering expense for the valves is disadvantageously high.